Rowling, who initially spoke of the "liberating experience" of adopting an alias, said in a statement: "I have today discovered how the leak about Robert's true identity occurred. A tiny number of people knew my pseudonym and it has not been pleasant to wonder for days how a woman whom I had never heard of prior to Sunday night could have found out something that many of my oldest friends did not know.

"To say that I am disappointed is an understatement. I had assumed that I could expect total confidentiality from Russells, a reputable professional firm, and I feel very angry that my trust turned out to be misplaced."

Russells told Rowling's agent when the truth emerged. The company said in a statement: "We, Russells Solicitors, apologise unreservedly for the disclosure caused by one of our partners, Chris Gossage, in revealing to his wife's best friend, Judith Callegari, during a private conversation that the true identity of Robert Galbraith was in fact JK Rowling.

"Whilst accepting his own culpability, the disclosure was made in confidence to someone he trusted implicitly. On becoming aware of the circumstances, we immediately notified JK Rowling's agent. We can confirm that this leak was not part of any marketing plan and that neither JK Rowling, her agent nor publishers were in any way involved."

The revelations have boosted sales however. Rowling's publishers have reprinted 140,000 copies to meet demand. It had shifted just 1,500 copies before her identity was uncovered.